A photolithographic process is generally used in the fabrication of semiconductor devices. In this process, a thin film of chemical photoresist is applied to the surface of a very thin crystalline material. A pattern on that surface is then exposed to ultraviolet light under controlled conditions. The exposed photoresist undergoes a chemical change which causes it to be either removed or retained when the crystalline material is later given a chemical bath. The pattern therefore defines an area on the crystalline material surface to be subjected to or protected from subsequent processing steps.
Demands are constantly being made for increased miniaturization of circuit design, with some structures presently required in the sub-micron line width region. The control of all process variables is therefore critical.
The principal variables in the photolithographic process are (1) the photoresist film thickness, (2) the exposure speed of the chemical photoresist, (3) the ultraviolet light intensity, uniformity and collimation during exposure, and (4) the exposure time. Photoresist film thickness is not difficult to control by viscosity and mechanical means, and stable photoresists are available with exposure speeds within a known and repeatable specification. Electromechanical timers of high accuracy are also available to control shutters to a predetermined exposure time. This invention involves the control of light intensity during exposure.
The control of light intensity from a high pressure mercury vapor lamp of the type used in this process cannot be accomplished by simply providing the lamp with a constant level of power. The intensity of those lamps decreases over time as the lamp interior becomes discolored from mercury and electrode material deposited thereon.
One approach to this problem has been to integrate the light intensity over time to arrive at an exposure time which will allow the desired amount of light to contact the photoresist. The exposure time is therefore increased as material deposited on the inside of the lamp envelope decreases light intensity. This longer time period results in unnecessary undercutting of the desired pattern due to chemical cross-linking of the photoresist molecules.
Another approach has been to attempt to maintain the light intensity constant by increasing the power to the lamp as the deposit of foreign matter takes place. The devices heretofore produced have utilized photodiodes as light sensors. Because a photodiode varies current in relation to lamp intensity, it is inherently ill-suited for this purpose. The light intensity range to which it can respond is limited by the current which it can safely carry. In fact, the photodiodes used can only sense light to a maximum intensity in the neighborhood of 2 or 4 mw/cm.sup.2 . It is for this reason that existing constant intensity light sources have light sensors placed behind mirrors specially coated to permit passage of only a portion of the lamp light. These mirrors not only lose some of the lamp energy, but are a source of considerable expense and error. The special coating is costly to apply and often has low spots of reflectivity not experienced in conventional silvered mirrors. The low spots are particularly noticeable in research and work involving extreme miniaturization.
Photodiodes used for this purpose are also larger than is desirable. Their size has a lower limit imposed by the requirement that they be able to take a relatively high maximum current. This would interfere with placement directly in the light path even if their intensity-sensing range were great enough.
Constant intensity light sources in this field have also caused damage to other sensitive electronic equipment. Highly sophisticated equipment such as mask aligners must be located adjacent to or be a part of the light source, yet existing equipment can send damaging spikes through the supply lines or electromagnetic fields through space toward that equipment.
Existing mercury arc lamp power supplies in this field also have not provided a minimum power mode to operate the lamp during long periods between exposures. The lamps are therefore operated continuously at high power levels, causing foreign matter to be deposited on the inside of the glass envelope at a needlessly high rate.